July 2010 IEEE P802.15-10-0578-00-0007

IEEE P802.15

Wireless Personal Area Networks

Project / IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Title / Final text for clauses 5.5.3 and 5.5.4
Date Submitted / [14th July, 2010]
Source / [Sang-Kyu Lim, Dae Ho Kim, Il Soon Jang, You Jin Kim, Tae-Gyu Kang]
[ETRI]
[Daejeon, Korea] / Voice: [ +82-42-860-1573]
Fax: [+82-42-860-5218]
E-mail: [
Re: / [Response to LB #50 comments]
Abstract / [This document describes the final text for clause 5.5.3 and 5.5.4 in D2 draft.]
Purpose / [To resolve the LB comments related to 5.5.3 and 5.5.4]
Notice / This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
Release / The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

Blue: new text (changes tracked toward sub-editor versions of D2 from 21st June 2010)

Pink: Editorial instructions

Crossed out: deleted

Orange: Replace with labels (hot links) in framemaker

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5.5.3 Flicker compensation mitigation and dimming

5.5.3.1 Flicker compensation mitigation

Replace the current text (See DCN 10/496/r0)

The deletion of the last sentence will be compensated through a new subclause in 6.9.6. See DCN 497/r1.

Flicker is defined as unexpected and unpredictable brightness fluctuations that are perceptible by the human eye and can be injurious to human health; therefore, this standard supports flicker compensation. All devices shall be compliant to all applicable regulations in regards to flicker.

Flicker is defined as the fluctuation of the brightness of light that can cause noticeable physiological changes in humans. This standard strives for the mitigation of flicker.

The next paragraph is moved here from 5.5.4 and the original composition slightly modified.

The maximum flickering time period (MFTP) is defined as the maximum time period over which the light intensity can be changing but the resulting flicker is not perceivable by the human eye [B37]. To avoid flickering in VLC, the brightness during each MFTP needs to be equal.

The flicker in VLC is classified into two categories according to its generation mechanism: intra-frame flicker and inter-frame flicker. Intra-frame flicker is defined as bit-pattern dependent brightness discrepancies within the data frame. Inter-frame flicker is defined as the average brightness discrepancy between the packet frame transmissions and the idle time between data transmissions. The details on the flicker compensation technologies are described in 5.5.4 and 6.9.6.”

5.5.3.1.1 Intra-frame flicker compensation mitigation (The resolution committee approved this new text in 5.5.3.1.1. DCN= 10/472/r0)

Intra-frame flicker compensation is accomplished by the use of one of the following mechanisms: Manchester encoding as specified in 6.6.4.2, 4B6B encoding as specified in 6.6.4.1, 8B10B encoding as specified in 6.7.3.2 or VPPM as specified in 6.9.6 and Annex F”.

5.5.3.1.2 Inter-frame flicker compensation mitigation (The resolution committee approved this new text in 5.5.3.1.1. DCN= 10/472/r0)

The compensation method used for inter-frame flicker is the transmission of an idle pattern between data frames whose average brightness is equal to that of the data frames. The idle pattern is not specified and it is allowable to use an idle pattern symbol rate other than that used for data transmissions to avoid in-band modulation domain interference.

5.5.3.2 Light dimming

Light dimming is defined as controlling the perceived brightness of the light source according to the user's requirement and is a cross layer function between the PHY and MAC. The details on the light dimming function of MAC layer are shown in 7.3.10.

5.5.3.2.1 OOK dimming

Since OOK modulation is always sent with a symmetric Manchester symbol, compensation time needs to be inserted into the data frame to adjust the average intensity of the perceived source as shown in Figure X. This process breaks the frame in sub-frames and each sub-frame can be preceded by a resync field that aids in readjusting the data clock after the compensation time. The data frame is fragmented into sub-frames of the appropriate length after the FCS has been calculated and the FEC has been applied

Figure x – OOK dimming

Figure x– OOK dimming

5.5.3.2.2 VPPM dimming

VPPM is a modulation scheme adapted for pulse width light dimming and offers protection from intra-frame flicker. The light intensity for the payload can be adjusted by adapting the pulse width of VPPM symbols. The light intensity for the preamble and header can be adjusted by inserting compensation symbols of the appropriate length and intensity before the frame.as shown in Figure z. The details on VPPM dimming are described in 6.9.6 and AnnexF.

Figure z – VPPM dimming

5.5.3.2.3 Idle pattern and adjustment time dimming

An idle pattern whose duty cycle variation results in brightness variation can be inserted between the data frames for light dimming, as shown in Figure 6. The adjustment time (which means "ON" and "OFF" time of a light source) can be also inserted into either the idle pattern or into the data frame, as shown in Figure 6, to reduce or increase the average brightness of a light source. An idle pattern can either be in-band or out-of-band as defined by the modulation domain spectrum and both types of idle patterns are supported in this standard. An in-band idle pattern does not require any change in the clock and can be seen by the receiver. An out-of-band idle pattern is typically at a much lower frequency than the optical clock rate and is not seen by the receiver.

However, the use of idle pattern for light dimming is in conflict with the inter-frame flicker compensation because that can be resulted in the inter-frame flicker. "ON" and "OFF" times of light source are essential to the dimming through the use of idle pattern, but they decrease the communication efficiency extremely. Therefore, the standard supports that the idle pattern is used only on the applications such as P2P communication which the flicker is allowed.

Figure 6 – Idle pattern and adjustment time dimming (Figure 6 depends on CID 110, so it can be modified a little bit. CID 296, 751. Figures throughout the document need to be adjusted after adding the new figures in this section)

5.5.3.2.4 Visibility Pattern dimming

Visibility patterns are in-band idle patterns that are specifically used in the payload of a visibility frame. The visibility patterns are used for supporting features such as flicker mitigation, continuous visibility, device discovery and color stabilization. The visibility patterns are not encoded in the PHY layer and do not have a FCS associated with them. In order to generate high resolution visibility patterns from 0% to 100% in steps of 0.1%, there are certain constraints that need to be used in the design criteria for visibility patterns.

1)  The number of transitions between 0's and 1's should be maximized to provide high frequency switching in order to avoid flicker and to help the CDR circuit at receiver for synchronization purposes, if used.

2)  Visibility pattern generation should be made in a simple manner. Designing a thousand patterns to support low resolutions (as low as 0.1% resolution) is not practical and makes visibility pattern generation and use very complex.

3)  Since visibility patterns are transmitted without changing the clock frequency (in-band), the patterns avoiding conflicts with existing RLL codewords are preferred.

The generation of the visibility patterns and their usage is defined in 6.9.6.2.2

5.5.3.2.5 Amplitude dimming

Amplitude dimming controls the brightness by changing the current driving the light source. However, a color shift of the light source may arise from the control of driving current for dimming and may not be recommended for illuminators.

5.5.3.3 Idle pattern

The first sentence is already shown in inter-frame flicker compensation (5.5.3.1.2) and idle pattern and adjustment time dimming (5.5.3.2.3). So, it has been deleted.

The idle pattern can be used to compensate for inter-frame flicker and to allow a light source to support dimming. However, the use of idle pattern for light dimming is in conflict with the inter-frame flicker compensation because that can be resulted in the inter-frame flicker. The on and off times of light source are essential to the dimming through the use of idle pattern, but they decrease the communication efficiency extremely. Therefore, the standard supports that the idle pattern is used only on the applications such as P2P communication which are independent of flicker and it allows to be used only for the inter-frame flicker compensation on the applications which need the non-flicker.

5.5.3.4 Idle Pattern

The idle pattern is related to both the inter-frame flicker compensation and the light dimming; therefore, the standard supports that the idle pattern is used only on the applications such as P2P communication which are independent of flicker and it allows to be used only for the inter-frame flicker compensation on the applications which need the non-flicker. The details on the idle pattern are described in 6.9.6.

5.5.4 Flicker mitigation (Informative)

5.5.4 has been partially merged with 5.5.3.1, and the remaining text deleted (see below).

Illumination flicker can be harmful to human health and is defined as unexpected and unpredictable light intensity change. Flickering can be caused by low rate repetition of the light source turning on and off or a slow change of brightness over a time period. There is a flickering in VLC lower data rate than 200 bps.

The maximum flickering time period (MFTP) is defined as the maximum time period over which the light intensity can be changing but the resulting flicker is not preceivable by the human eye [B37]. To avoid flickering in VLC, the brightness of each MFTP needs to be all equal. (This paragraph has been moved into 5.5.3.1)

A solution for flicker removal during ampltidue modulated data transmission maintaining a constant ratio of positive and negative levels per MFTP. This can be done with the use of run length limiting codes as specified in 6.6.4. , 2 PPM, and 4PPM. Manchester code has 50% duty cycle always. 2 PPM is 50% and 4 PPM is 25%. If we use this RLL code or modulation scheme for data stream and at idle time we use same waveform, flickering will not occur. But if we use NRZ OOK, we need another solution. Second solution is we make a ratio of 1 and 0 per MFTP to be constant at data stream.

To prevent the LED from appearing “dimmer” during the packet frame transmission time, an idle pattern is sent between frames that has the same duty cycle as the modulated frame. The pulse repetition rate can be set lower so as not to cause in-band modulation domain interference with any VLC data modulations.

TG-VLC Submission Page XXX Sang-Kyu Lim <ETRI>